Wavelength division multiplexing permits transmission of a large number of different optical channels in a lightwave communication system. In order to accomplish this, it is generally necessary to have a plurality of light sources wherein each light source is tuned or set to a particular operating wavelength different from the operating wavelength for the other light sources. That is, no two light sources in the wavelength division multiplexed system share the same operating wavelength. Tunable single frequency (wavelength) lasers such as distributed feedback and distributed Bragg reflector lasers are sufficiently versatile to meet the requirements necessary for being light sources in a wavelength division multiplexed lightwave communication system. For example, see IEEE J. of Quantum Elec., Vol. QE-23, No. 6, pp. 903-8 (1987).
Tunable distributed Bragg reflector (DBR) lasers have electrically isolated contacts so that one current can be applied to the Bragg section while a separate current can be independently applied to the gain section. For example, see Elect. Lett., Vol. 24, No. 23, pp. 1431-3 (1988) showing a three-section continuously tunable DBR laser having Bragg, phase, and gain sections. By applying a predetermined current to the Bragg section, it is possible to tune the Bragg grating and therefore the Bragg wavelength, .lambda..sub.B, to a wavelength which corresponds to a wavelength of one of the longitudinal or Fabry-Perot modes of the laser. By utilizing this technique with a plurality of tunable DBR lasers, one could obtain a corresponding plurality of individually mutual exclusive wavelengths, each for a separate channel, so that wavelength division multiplexing can be achieved.
As the tunable DBR lasers in the wavelength division multiplexing application described above are run for a long period of time, the devices degrade in performance because of aging and material defects. As a result, the Bragg reflection characteristic drifts in wavelength. In other words, the effective period for the Bragg grating drifts or changes. The wavelength drift of the grating can be large enough to cause a "mode hop" to occur wherein the originally lasing longitudinal mode is discriminated against by the Bragg grating in favor of another mode which was formerly a side longitudinal mode of the original lasing mode. In the wavelength division multiplexing application, this would cause a channel experiencing a mode hop to appear at a wavelength, and therefore a different longitudinal mode, from that originally assigned.